mathematical problem

Four consecutive natural numbers must be two odd numbers, two even numbers, the sum of four reciprocal numbers is 19/20, and the prime factor of denominator 20 is 2*2*5. Of the four natural numbers, 5 must be an odd number, and the other odd number may be 3 or 7. If the other odd number is 3, we can try.

19/20- 1/5- 1/4- 1/3 = 57/60- 12/60- 15/60- 20/60 = 10/60 = 1/6

Obviously, these four natural numbers are 3, 4, 5 and 6.

Third question

Considering the work as a whole 1, the efficiency of Party A plus the efficiency of Party B is 1/4, and the efficiency of Party B plus the efficiency of Party C is 1/5. Now Party A and Party C will cooperate for 2 hours, and Party B will do the rest for 6 hours alone. It can be regarded as1-/kloc-0 after two hours of cooperation between Party A and Party B and two hours of cooperation between Party C..

the second question

Let's list some of these natural numbers first, and we will see them.

10 1 12358、 1 12358、 12358、 1347、 1459、 156、 167、 178、 189

202246、2 1347、2246、2358、246、257、268、279

…………

As long as the first two digits do not add up correctly, it is such a natural number.

According to 9+0= 8+ 1= 7+2= 6+3= 5+4,

The first digit is 1, the second digit is 0 to 8, and the number is *** 9.

The first digit is 2, the second digit is 0 to 7, and the number is *** 8.

The first digit is 3, the second digit is 0 to 6, and the number is *** 7.

The first digit is 4, the second digit is 0 to 5, and the number is *** 6.

……